The metabolic changes accompanying diabetes mellitus in nonhuman primates are useful for studying the development and progression of vascular diseases. Hyperglycemia, hyperlipidemia, and impaired insulin secretory capacity of Macaca nigra lead to secondary complications-- cataracts, atherosclerosis, and muscle microangiopathies--the severity of which relates to the clinical history of each monkey. A major cause of the diabetic syndrome is a nonselective infiltration of amyloid into the islets of Langerhans with a consequent loss of both beta and alpha cells; both the pathology and the possible relationship of this filtration arising from an autoimmune reaction are to be examined. Glucagon, cortisol, and catecholamines are all involved in glucose and lipid metabolism. Assays for these hormones are being established to provide a better understanding of the metabolic aberrations that lead to vascular lesions. Measuring only the periphery yields an incomplete understanding of the specific metabolic events. Several infusion studies are planned whereby indwelling catheters in various loci within the monkey can be monitored simultaneously. Finally, some studies will be done to determine whether a viral element is operative in the diabetes affecting these monkeys. Bibliographic references: Howard, C.F. Jr. Basement membrane thickness in muscle capillaries of normal and spontaneously diabetic Macaca nigra. Diabetes 23:201-216, 1975. Howard, C.F. Jr., and Palotay, J.L. Spontaneous diabetes mellitus in Macaca cyclopis and Mandrillus leucophaeus: case reports. Lab. Anim. Sci. 25:191-196, 1975.